In the evanescent illumination of a sample, the ray bundle for illuminating the sample is supplied in such a way that total reflection occurs on the boundary surface to the sample. A standing evanescent wave is thus formed in the sample, the intensity of which decreases exponentially with the distance to the boundary surface.
The distance at which the intensity has dropped to the 1/eth part can be defined as the distance which describes the penetration depth of the evanescent wave. Typical values lie in the range of 100 to 200 nm.
The evanescent radiation is used for exciting fluorescence in the sample for example. The excited fluorescence radiation can then be detected by means of a microscope.
Increasingly, several fluorescences with different wavelengths are to be excited. This should occur under the same conditions to the highest possible extent, so that the individual fluorescence images remain meaningful and can be combined with one another or compared with one another quantitatively. The difficulty occurs in this connection that the penetration depth of the evanescent illumination depends on the wavelength and the incidence angle. The penetration depths are different for different wavelengths at the same incidence angle.